What is SFRA?
Sweep Frequency Response Analysis.
Doble’s M5400 Sweep Frequency Response Analyzer is the most reliable and robust SFRA system in the world.
Sweep Frequency Response Analysis (SFRA) is a tool that can give an indication of core or winding movement in transformers.
This is done by performing a measurement, albeit a simple one, looking at how well a transformer winding transmits a low voltage signal that varies in frequency.
Just how well a transformer does this is related to its impedance, the capacitive and inductive elements of which are intimately related to the physical construction of the transformer.
Changes in frequency response as measured by SFRA techniques may indicate a physical change inside the transformer, the cause of which then needs to be identified and investigated.
The SFRA results for each phase of the transformer are plotted as dB responses against frequency.
In this case, the response of one phase is clearly very different from the other two, and the form of difference indicates a shorted turn in this case.
It is important to get good resolution in results such as this to give clear and unambiguous traces at low frequencies.
Power Transformers and Mechanical Integrity.
Power transformers are specified to withstand the mechanical forces arising from both shipping and subsequent in-service events, such as faults and lightning.
Transportation damage can occur if the clamping and restraints are inadequate; such damage may lead to core and winding movement.
The most severe in-service forces arise from system faults, and may be either axial or radial in nature.
If the forces are excessive, radial buckling or axial deformation can occur.
With a core form design the principal forces are radially directed, whereas in a shell form unit they are axially directed, and this difference is likely to influence the types of damage found.
Once a transformer has been damaged, even if only slightly, the ability to withstand further incidents or short circuits is reduced.
There is clearly a need to effectively identify such damage.
A visual inspection is costly and does not always produce the desired results or conclusion.
During a field inspection, the oil has to be drained and confined space entry rules apply.
Since so little of the winding is visible, often little damage is seen other than displaced support blocks.
Often, a complete tear down is required to identify the problem.
An alternative method is to implement field-diagnostic techniques that are capable of detecting damage, such as SFRA.
There is a direct relationship between the geometric configuration of the winding and core within a transformer and the distributed network of resistances, inductances, and capacitances that make it up.
This RLC network can be identified by its frequency- dependent transfer function.
Changes in the geometric configuration alter the impedance network, and in turn alter the transfer function.
Changes in the transfer function will reveal a wide range of failure modes.
SFRA allows detection of changes in the transfer function of individual windings within transformers and consequently indicate movement or distortion in core and windings of the transformer.
What is the motivation for SFRA Measurements?
There are two distinct environments for application of sweep frequency response measurement: in the factory and in the field.
In both cases the procedures and precautions used to generate a good measurement are the same.
However, there is a difference in motivation for the tests in each category.
How to use SFRA in Factory Application?
Reasons to use SFRA in a factory environment include:
- Quality assurance.
- Baseline reference.
- Relocation and commissioning preparation.
Manufacturers are using SFRA as part of their quality program to ensure transformer production is identical between units in a batch.
The accuracy and repeatability of SFRA are key to the program; the range from 20 Hz to 2 MHz is required to diagnose variations related to the core, the clamping structure, windings and leads.
An SFRA baseline can be produced in the factory when the transformer has been filled with oil and dressed as part of the factory commissioning tests.
Many customers now appreciate the benefits of having a good baseline for SFRA measurements in the field when they need to respond to an incident.
These customers require an SFRA measurement as part of their transformer purchase specification.
There are cases where a transformer is also tested in the factory without oil immediately prior to transport to the customer site.
Some utilities specify that the transformer is shipped with small test bushings fitted to allow this test to take place.
This allows the transformer to be tested as soon as it arrives on site without costly dressing and oil processing procedures.
SFRA is safe to perform on a suitably prepared transformer without oil as the test is low voltage one.
How to use SFRA in Field Application?
Reasons to use SFRA in a field environment include:
- Relocation and commissioning validation
- Post incident: lightning, fault, short circuit, seismic event etc
Once a transformer arrives on site after relocation it may be tested immediately, without oil if required, for comparison with baseline references or with sister units. (The provision of small test bushings prior to shipping aids in testing).
This gives confidence in the mechanical integrity of the unit prior to commissioning.
Some utilities prefer to check the impact recorders after the relocation and then, assuming no adverse impact recorder results are found, redo factory based SFRA tests once the transformer is dressed and ready for commissioning.
After a close up fault, or as a result of concern about the transformer from, for example, rising DGA levels, SFRA is a key tool in the engineer’s toolbox for diagnosing the health of the transformer and its suitability for service.
There is much to be gained in terms of information about mechanical integrity from an SFRA measurement, which supports evidence from Power Factor and Capacitance testing, Transformer Turns Ratio and Winding Resistance measurements.
Building a complete picture of the transformer from all available data is critical in making engineering judgements about an individual unit.
Returning an unhealthy unit to service may prove catastrophic.
Which measurements are made with SFRA?
The most useful SFRA measurement is the response of individual winding sections of the transformer at different frequencies.
This allows problems to be associated with individual winding sections, rather than on a phase or winding generally.
The SFRA does this in a simple way by injecting a signal of known frequency into one end of the winding and measuring the response at the other.
By sweeping through the frequency range of interest, from just above DC to several MHz, it is possible to make accurate, repeatable and reliable measurements.
SFRA measurements are independent of the lead arrangement and of the measuring device up through the frequencies of interest.
For tests on a large power transformer 20m leads are needed which give reliable and repeatable results up to the MHz range.
To make an SFRA measurement, a transformer must be prepared as it would for power factor and capacitance measurements.
Each winding section, HV and LV, is analyzed separately.
Transfer function measurements, from HV to LV, may also be made, but these are less effective at detecting movement.
Short circuit SFRA measurements, made for example by performing an SFRA on a HV winding with the LV winding shorted, provides further information about winding integrity and relates to transformer Leakage Reactance.
The SFRA provides a wealth of information in the form of a frequency response plot, which needs to be interpreted.
Doble Engineering provides both a measurement and an interpretation service, relying on experts in the technique, and referencing a library of transformer SFRA results.
What does an SFRA measurement look like?
An SFRA measurement, in simple terms, is a Bode plot, a measure of response against frequency.
Response could be measured in Volts, but is usually measured in decibels (dB’s) to relate the output to the input.
High-voltage winding measurements have greatest attenuation as compared to the other categories.
Most traces start between –30 dB and –50 dB and are initially inductive.
High-voltage windings are much larger in overall size, which contributes to greater complexity in its distributive network.
High-voltage winding measurements generally produce steeper resonances and more of them as compared to its low-voltage counterpart.
What M5400 from Doble offers?
Doble’s M5400 Sweep Frequency Response Analyzer is the most reliable and robust SFRA system in the world.
It offers state of the art hardware and software that produces repeatable results across the frequency range of interest, supported by world class engineers to assist with test procedures and result interpretation.
The most important aspect of frequency response analysis is to get valid results from a particular test for decision making purposes.
This means that the results reflect the true response of the windings, and do not include associated test lead or equipment affects. After years of study and research, Doble has chosen the Sweep Frequency method for making the measurement, as other approaches have not yielded cost-effective systems or reliable results.
Doble offers the M5400 and the full support and service associated with Doble products.
Results may be analyzed and interpreted using Doble’s in-house experts who are able to reference an extensive library of SFRA results built up over years of testing a wide variety of transformers.
The Doble M5400 has a resolution that is logarithmic, remaining constant at 1.2% at every frequency.
This means that resolution is maintained across the frequency range. The Doble M5100 software allows data to be displayed either in a log scale format or in a linear scale format.
The Doble M5400 gives consistent and reliable results from 10 Hz to the MHz range, allowing for interpretation in terms of key elements within the transformer: core, clamping structures, main and tap windings, internal support leads.
Without this full range of frequency coverage, as provided by the Doble M5400, the test results are compromised.
The Doble M5400 software allows for display of the whole of the measurement made, or through an intuitive graphical interface allows particular sections to be viewed in detail.
In addition the software displays sub-bands for those who are more comfortable viewing data in that way.
Sweep Frequency Response Analysis is a powerful tool for use in analyzing transformer health and mechanical integrity.
It has proven value in the field and factory, as indicated in the case studies given here.
The Doble M400 test instrument produces reliable, robust and repeatable results.
These cover the full range necessary to make transformer health diagnoses relating to the core, the windings and the tap changer.
The Doble M5400 SFRA test set is a vital tool for today’s engineer.